I have a simple timer interrupt based GPIO toggling code to be tested on STM32F401 discovery board as shown in this link http://codepaste.net/fhmh2v . The problem is this code doesn't toggle the LED/GPIO. I set a breakpoint inside the timer handler and I observed it is only hit once (I clear the interrupt flag in ISR) and then the whole program is trapped in the while loop of the main function. Could anyone suggest what can be wrong? Thanks.